The present invention relates to the control of the printing elements of impact printers such as dot matrix printers operating at very high data rates. In particular, the invention relates to control of the driving force applied to the printing element.
This invention applies to printers which develop a force whose direction is a function of the direction of the driving pulse. Generally, these include a plurality of printing elements, each having a moving electromagnetic coil operating in a fixed magnetic field, having attached thereto a print wire or stylus, the styli being arranged in spaced-apart relationship in a print head. One such print head is disclosed in U.S. Pat. No. 4,129,390, granted to J. E. Bigelow et al on Dec. 12, 1978, and comprises a stacked array of flat blade type printing elements.
Each printing element or blade is normally biased against a backstop and has associated therewith a drive circuit for controlling the operation thereof. The drive circuit applies a drive pulse to the printing element for moving it toward a record medium for printing indicia, the printing element then returning to the backstop. In order to minimize rebound of the printing element from the backstop, a damping pulse of reversed current may be applied to the printing element. In copending Application U.S. Ser. No. 204,628, filed concurrently herewith, by W. A. Hanger and A. B. Carson, entitled "BOUNCE CONTROL SYSTEM FOR IMPACT PRINTER", and assigned to the assignee of the present invention, there is disclosed a circuit for applying such a damping pulse to the printing element in response to impact of the printing element on the backstop, the damping pulse having a polarity opposite to that of the drive pulse. In that system, if a drive pulse overlaps in time with a preceding damping pulse, the two are simply additively supplied to the printing element, effectively cancelling each other out in the region of overlap.
A fundamental problem encountered in impact printers is the variation in printing rates. Because the timing between dots varies, the initial printing element momentum is not constant. This is because at high print rates it is necessary to apply a drive pulse to the blade before the kinetic energy for printing the preceding dot has been fully dissipated. It is, therefore, desirable to provide a variable period or current for the drive pulse to compensate for the differences in printing element starting momentum in order to achieve uniformity of print intensity.
U.S. Pat. No. 4,162,131, issued to A. B. Carson, Jr. et al on July 24, 1979, discloses means for modulating the amplitude or width of the drive pulse as a function of the time interval between successive drive pulses. But that system does not utilize damping pulses and, therefore, cannot take account of the effect of damping pulses on the overall energy supplied to the printing element.